v.] THE FRESH-WATER MUSSEL. 335 



y. The exoskeleton. Observe that this forms, during 

 life, a continuous investment for all the exposed 

 surfaces of the body, with the exception of the 

 foot. It consists of a pair of calcareous valves^ 

 which pass — dorsally into a cortiified ligament — 

 ventrally into a chitinoiis free-border^ which is in- 

 flected on to the edges of the pallial muscles. 



8. The branchice; two pairs of lamellate organs, 

 confluent with each other and the body-wall 

 internally and with the pallial-lobes externally. 

 Examine their central cavities and attachments, 

 noting especially the subdivision of the supra- 

 branchial chamber into two, by the suspensory 

 ligament of the gills. (Cf Sect. B. d, f3.) 



€. The body-cavity. This is almost obliterated by 

 the great development of the reproductive gland; 

 note the large pericardial chamber^ situated in the 

 dorsal middle line. 



'C,. The viscera. The cut edges of the i?itestine will 

 be seen, lying within the body and traversing the 

 pericardial chamber. 



The excretory organ ; median and paired, lying 

 immediately beneath the pericardial chamber. 

 The glandular segments of opposite sides are 

 seen to be approximated in the middle line; 

 note the reno-pericardial apertures lying imme- 

 diately above and internal to the attachments of 

 the inner gill lamellce. The muscular segments 

 are here confluent in the middle line, enclosing 

 a spacious cavity ; dissect to the level of the 

 excretory orifice of one side, and note that it is a 

 perforation of the floor of this segment, opening 



